A green mist swirls beneath the surface of the once crystal waters of Walden Pond, threatening to turn one of America's most iconic lakes into a slimy, scummy mess.

Humans are predictably to blame for Walden's deterioration: Decades of careless tourism have led to an explosion of algae in the water, in what scientists warn could irreversibly alter the life of the pond. Now, it appears that climate change is hastening its decline.

Perhaps one of the first questions should be: Why does it matter? A murky pond could easily be dismissed as a trifling issue in an age of prolonged drought, poisoned drinking water, and serious storms.

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But in the darkening waters of Walden Pond, scientists observe more than the loss of a lake: They see the decline of one of America's most important cultural and literary locations. It was at Walden Pond that the writer and philosopher Henry David Thoreau set up his cabin in the 19th century, in an attempt to "live deep and suck out all the marrow of life." He documented his experiences in Walden; or, Life in the Woods,and the pond has since become a symbol of natural purity, ecological consciousness, and the high-minded simple life.

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Though he chose Walden Pond for this experiment, Thoreau did not find the surroundings transcendental, precisely. "The scenery of Walden is on a humble scale, and, though very beautiful, does not approach to grandeur," he wrote. What he did find remarkable, however, was the "depth and purity" of the lake, a "deep green well" that was formed by the remnants of underground slab of glacial ice deposited in New England around 15,000 years ago.

In the 1920s, Walden Pond started to become a popular recreational site for swimmers and tourists, who were drawn to its literary and philosophical significance, as well as to its natural beauty and crystalline waters (plus its proximity to Boston).

The area was established as a state reservation in 1922, and beach and bath-house facilities were built on the eastern shore. The construction of a new footpath to Thoreau's cabin, alongside deforestation and landscaping, washed large volumes of soil into the lake. Sport-fishers were lured when local officials stocked the pond with non-native species, including rainbow and brown trout, replacing the pond's original fish. Swimmers marked their territory by urinating into the lake.

All this led to a massive uptick in nutrients flowing into Walden Pond, causing the number of algae to grow. Over time, the water became murkier and murkier.

"We've basically been loving the lake to death for the last century," says Curt Stager, the lead author of a new study, published in PLoS One, looking at the environmental changes to Walden Pond, and an expert in lakes at Paul Smith's College.

These changes are recorded in the layers of sediment underneath the pond. With the help of half a dozen of his students, Stager extracted sediment cores from the lake, which he then examined under a microscope for the remains of diatoms—a certain type of algae that are naturally enclosed in microscopic glass boxes. These resist the process of decay, and can thus provide a peek into the past.

What would look like a pile of mud to a layperson looks more like the pages of a history book to Stager.

"If you dive to the bottom of Walden Pond—which some people do, there are free divers—if you push your hand in all the way up to the wrist, you'll touch the layers of plankton that Thoreau swam among in the mid-1800s," Stager says. "If you shove it in just a little less, the length of all your fingers, you'll hit a layer of radioactivity from the atomic bomb testing in the Pacific. We're looking at the youngest layers now, with an eye to the future."

Thoreau himself left a trail of clues about the environmental history of Walden Pond. As well as being a writer and philosopher, Thoreau was a keen scientist and record-keeper, mapping the bottom of Walden Pond and keeping track of seasonal changes in the surrounding area of Concord, Massachusetts.

Thoreau recorded many of these observations in Walden, but he kept other scientific records as well. Richard Primack, a professor of biology at Boston University, was the first to come across this further evidence of Thoreau's work as a scientist. Sitting unpublished in New York's Morgan Library were around 15 years' worth of Thoreau's handwritten spreadsheets, chronicling everything from when Walden Pond's winter ice would thaw, to when the flowers around Concord would come into bloom.

It was a kind of "calendar of natural history," Primack says, based on which he's been able to make various observations about contemporary environmental change—including the fact that, thanks to climate change, the ice on Walden Pond now thaws about two weeks earlier than it did in the past.

Geologically speaking, there are tens of thousands of lakes across the United States that are similar to Walden Pond, but in terms of mystique and literary significance, Walden remains unique. That is reason enough for its preservation, Primack says.

"As a biologist, I have to think about protecting endangered plant and animal species, but there are a lot of places that are protected for their beauty, for their cultural significance," he says. "The most striking thing about Walden Pond is that it's protected because of a work of literature, because of the person who lived there while writing this work of literature. What's amazing is that the significance of the work is growing all the time."

Stager has found another benefit of the perpetual allure of Walden Pond: an opportunity to reach a new generation. Stager has worked with writer David Press, a retired English professor at PSC, and artist Emilyann Cummings to transform his scientific endeavors into a graphic novel, in which he and his students feature as characters alongside Thoreau himself, explaining the threats to Walden Pond. It is distributed freely to local schoolchildren.

Selections From 'Walden: The Graphic Novel'

Curt Stager worked alongside writer David Press and artist Emilyann Cummings to create a graphic novel, based on his work about Walden Pond. The comic was funded by Stager's National Science Foundation grant.

The good news is that the Massachusetts Department of Conservation and Recreation, which has managed the area since 1975, agrees that Walden Pond is worth preserving. MDCR has implemented various measures over the years to prevent further deterioration, including shoreline stabilization and restoration efforts, plus the closure of a nearby landfill, which had the happy side effect of stopping gulls from pooping in the lake.

Nonetheless, it appears that the pond may already have been irrevocably changed, and some of its potential contributions to the world of natural history have been lost forever. Thoreau noted, for instance, that the fish in Walden Pond appeared unusual compared to other fish in the region. They’ve gone now, and scientists will never find out if the lake once harbored a unique variety of fish.

Furthermore, it looks like climate change is starting to have an unwelcome and unexplained hand in the growing murkiness of the pond, introducing a new kind of plankton to the water. Scientists also worry that warmer weather will mean more day trippers from Boston in the future.

"It will be a double hit," Stager says. "To keep Walden Pond as beautiful and clear as it now is, we're going to have to work even harder just to hold it steady."

New Landscapes is a regular series investigating how environmental policies are affecting communities across America.